This invention relates to a continuous casting apparatus, and more particularly to an electromagnetic brake device for a continuous casting mold which applies braking to a flow of molten steel from an immersion nozzle in the continuous casting of steel, thereby reducing inclusions contained in the molten steel.
Japanese Patent Unexamined Publication No. 63-203256 discloses a technique directed to an apparatus for decelerating a flow of molten steel from an immersion nozzle in a casting mold to reduce inclusions contained in the molten steel.
As shown in FIGS. 1 and 2, in this technique, two pairs of magnetic poles 12 of an electromagnetic brake are disposed locally laterally of a molten steel discharge flow path of an immersion nozzle 29. Electromagnets 11 used here have a laterally-elongated horse shoe-shape in a horizontal cross-section, and coils 28 are wound respectively on the opposite ends of each electromagnet, and these portions serve as the magnetic poles 12.
As shown in FIG. 3, the magnetic poles 12 are inserted respectively into openings 33 provided in long-side water boxes 2 of the mold, and are extended through long-side backup plates (not shown), and the end faces of the magnetic poles are secured to long-side copper plates 3 by bolts, thereby mounting yoke portions 13 of the electromagnets 11 on the long-side water boxes 2. The long-side water box 2 is mounted on a mold support frame 35 through support shafts 34 mounted on the opposite ends of this box. The mold support frame 35 is mounted on vibration tables 8. The long-side backup plate, covering a region 0.5 to 2 times larger in size than each side of the magnetic pole with reference to the center of the magnetic pole, is made of a magnetic material.
In FIGS. 1, 2 and 3, 4 denotes a short-side backup plate, 5 denotes a short-side copper plate, 26 denotes a cast piece, 29a denotes a molten steel outlet, 30 denotes the molten steel, and 40 denotes lines of magnetic force.
In the technique disclosed in the above Japanese Patent Unexamined Publication No. 63-203256, since the magnetic poles 12 are disposed locally along the molten steel discharge flow path of the immersion nozzle 29 as shown in FIG. 4A, the discharge flow, after passing past the magnetic field, hardly becomes a uniform flow, as shown in FIG. 4B, and the inclusions in the molten steel flow are involved internally to intrude deep into the molten steel, so that the effect of reducing the inclusions can not be sufficiently expected.
The electromagnetic brake device has a considerable weight, and in the type of construction fixedly mounted in the casting mold, this device vibrates together with the casting mold during the operation, and therefore it is necessary to firmly fix the device to the casting mold. As a result, particularly when grounding the device to an already-installed continuous casting apparatus, there are needed considerably extensive modifications of the equipment, such as an increased outer size due to a rigid construction of the casting mold, an increased motor capacity due to an increased load on the mold vibrating device, and an increased strength of a drive system necessitated by it. Therefore, much cost is required for the modifications, and there are encountered many drawbacks such as an installation difficulty.
Japanese Patent Publication No. 49-30613 discloses a technique in which magnetic poles of an electromagnet are disposed outside a casting mold, and windings are wound on them, and the magnetic poles are connected together by yokes to provide an integral construction. Lines of magnetic force pass through molten steel poured into the casting mold. However, in the technique disclosed in the above Japanese Patent Publication No. 49-30613, the width of the magnetic pole is small relative to the width of the casting mold, and as a result a sufficient magnetic flux density is not produced at the widthwise ends of the casting mold, and inclusions are inevitably involved internally to intruded own ward, so that the effect of reducing the inclusions can not be sufficiently expected.
For example, Japanese Patent Unexamined Publication No. 1-271031 proposes, as a method of producing a composite steel material by continuous casting equipment, a technique in which an electro-magnetic brake device is mounted within a continuous casting mold. In this technique, two immersion nozzles of different lengths are used, and an electromagnet is provided between molten steel injection portions of these immersion nozzles, and a double-layer composite cast piece in which the boundary between the surface layer portion and the inner layer portion is made clear by magnetic means is obtained.
However, a specific construction of the electromagnetic brake device for such a continuous casting mold has never been disclosed or reported so far.
Japanese Patent Unexamined Publication No. 1-99763 describes that in the technique disclosed in the above Japanese Patent Unexamined Publication No. 63-203256, the magnetic flux density necessary for decelerating the molten steel flow from the immersion nozzle to reduce the inclusions contained in the molten steel is 2500 to 3500 Gauss.
However, with respect to this technique, when a composite steel material is to be produced using two immersion nozzles, it is necessary to separate the molten steel for a surface layer, injected from the nozzle at the upper portion of the casting mold, from the molten steel for an inner layer injected from the lower nozzle. Therefore, there is needed the magnetic flux density which is uniform over the entire width of the cast piece and has a value about twice larger than the above-mentioned value. As a result, the outer shape of the electromagnet becomes larger than that of the casting mold, and an available installation space is limited by the provision of peripheral devices such as a tundish-car and a mold vibrating device, which results in a problem that the installation becomes impossible. Further, generally, in the production of a composite steel material, metal used for a surface layer has higher quality and more excellent properties, such as corrosion resistance and wear resistance, than metal for an inner layer. From the viewpoint of the production cost, it is important to obtain the optimum thickness of the surface layer metal. Further, in the casting of the double-layer cast piece, if the immersion nozzle for the inner layer metal is too long, it becomes clogged during the use, and also due to troubles such as one that it is liable to be broken, a durability problem is encountered. For these reasons, it is most preferred that the electromagnetic brake device should be mounted within the casting mold. However, in this case, for the above-mentioned reasons, there is encountered a problem that in practical use, it is impossible to install it.